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Photocatalytic degradation of nitrobenzene by gold nanoparticles decorated polyoxometalate immobilized TiO2 nanotubes
Separation and Purification Technology 171 (2016) 62–68
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Photocatalytic degradation of nitrobenzene by gold nanoparticles decorated polyoxometalate immobilized TiO2 nanotubes Ali Ayati a,b,⇑, Bahareh Tanhaei b, Fatemeh F. Bamoharram c, Ali Ahmadpour d, Philipp Maydannik a, Mika Sillanpää a,e a
Laboratory of Green Chemistry, LUT School of Engineering Science, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland Department of Chemical Engineering, Quchan University of Advanced Technology, Quchan, Iran Department of Chemistry, Mashhad Branch-Islamic Azad University, Mashhad, Iran d Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran e Department of Civil and Environmental Engineering, Florida International University, Miami, FL 33174, USA b c
a r t i c l e
i n f o
Article history: Received 15 February 2016 Received in revised form 19 June 2016 Accepted 16 July 2016 Available online 18 July 2016 Keywords: TiO2 nanotube Polyoxometalate Gold nanoparticle Nitrobenzene Photodegradation
a b s t r a c t In this study, the gold nanoparticles were deposited on the surface of tungstophosphoric acid immobilized TiO2 nanotubes, in which tungstophosphoric acid plays the roles of highly localized UV-switchable reducing agent and multifunctional photocatalyst linker molecule. The prepared novel nanocomposite was characterized with FTIR, XRD, EDX and TEM and showed high photocatalytic efficiency in nitrobenzene removal. The results demonstrated that by introducing of AuNPs, the photocatalytic performance significantly enhanced, where the photocatalytic rate of using Au/HPW/TiO2-NTs nanocomposites was 4.1-fold increase compared to TiO2 nanotubes. The photocatalytic mechanism was proposed. Ó 2016 Published by Elsevier B.V.
1. Introduction Nitrobenzene (NB) as a highly toxic aromatic compound, is frequently discharged from the petroleum industry, dyes, herbicides, fungicides, wood and explosives manufacture effluents [1–3]. It is a skin and eyes irritant and affects the central nervous and cardiovascular systems and also, is suspected to be possible carcinogenic or mutagenic agent [3]. The maximum allowable concentration is reported 1 mg L1 in wastewaters [4]. In this regard, several treatment processes were reported for the nitrobenzene effective removal, such as ozonation [5], adsorption [6], catalytic [1] and photocatalytic degradation [7]. Among these methods, the latter process is largely studied due to the complete mineralization of organic pollutants to nontoxic or significantly less toxic inorganic compounds. Design of effective hybrid materials using the semiconductors has attracted growing interest for photocatalysis [8,9]. Titanium dioxide (TiO2) is one of the most widely studied semiconductor which has unique chemical and physical properties including its ⇑ Corresponding author at: Laboratory of Green Chemistry, LUT School of Engineering Science, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland. E-mail addresses: [email protected], [email protected] (A. Ayati). http://dx.doi.org/10.1016/j.seppur.2016.07.015 1383-5866/Ó 2016 Published by Elsevier B.V.
availability, low cost, nontoxicity, small crystal size, high specific surface area, highly porous structure [10–12]. TiO2 is well-known to exhibit photocatalytic activity due to the photo-generated charge carriers [10,13]. It is demonstrated that the TiO2 nanotubes as nanoparticulated forms of TiO2 possess higher photocatalytic activity in comparison to colloidal forms of TiO2, due to large specific surface area and pore volume [10,14] that can significantly increase the number of reaction sites. The most important drawbacks of bulk TiO2 and also, TiO2 nanotubes are their large band gap (Eg: 3.0–3.2 eV) which requires its exposure to ultraviolet irradiation for potential photocatalysis applications and its massive recombination rate of photogenerated electron(e)–hole (h+) pair which hinders its photocatalytic efficiency [15,16]. It is of great interest to develop TiO2 based photocatalyst with improved photocatalytic activity. Doping the TiO2 surface with noble metals, which act as electron acceptors, is one effective strategy to improve its photocatalytic activity. Among the noble metals, gold NPs deposition has attracted significant interest [17,18]. The charge separation between the excited electron (e) and hole (h+) resulting the strong interaction between gold and TiO2 [19] which decreases the band gap energy of TiO2 and enhances its photocatalytic activity [18,20]. On the other hand, it is demonstrated that the combination of the Polyoxometalate (POMs) and TiO2 can enhance the
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photocatalytic efficiency of TiO2-catalyzed reaction, due to the synergistic photocatalytic effect between these two components [21,22]. POMs as a molecularly defined inorganic metal–oxide clusters [23] can be easily reduced by photo/electro chemical procedures and their structures remain unchanged under stepwise and multielectron redox reactions. Because of these properties, they have been used as efficient photocatalysts, mild reductants and stabilizers in the green synthesis of metal NPs [24–27]. In our recent study [28], we applied Preyssler acid, as a POM structure, for the locally deposition of gold NPs on the TiO2 surface, wherein it played the role of interlink between the AuNPs and TiO2. The local decoration of AuNPs on the TiO2 surface prevents the formation of free non linked AuNPs in the solution and also, the formation of a novel nanocomposite composed of AuNPs, TiO2 and POM. In this study, we investigated the deposition of AuNPs on the tungstophosphoric acid (H3PW12O40, HPW) immobilized TiO2 nanotubes surface. The HPW (a Keggin type of POM) have attracted significant attention in catalysis and photocatalysis due to its low cost, low environmental impact, interesting electron and proton transfer, high thermal stability and storage characteristics [23]. We studied the photocatalytic activity of prepared nanocomposite for the degradation of nitrobenzene under simulated visible light irradiation.
source, under continuous stirring for an hour. The color of suspension was changed from milky to purple, indicating the formation of gold NPs. The obtained nanocomposite was separated using centrifugation (4000 rpm), washed with water and dried at 60 °C for 6 h.
2. Experimental
3.1. Mechanism of nanocomposite formation
2.1. Chemicals and instruments
The present HPW molecules bonded to TiO2 NTs in the HPW/TiO2-NTs composites were used for local reduction of gold ions on the surface of TiO2 NTs. By the UV irradiation of HPW/TiO2-NTs composites in the presence of 2-propanol, the bounded [PW12O40]3 can be easily reduced to [PW12O40]4 by the following equation [30–32]:
Titanium dioxide (TiO2, anatase, 99.8%), Tetrachloroauric acid (HAuCl4H2O, P99.9%), HPW and nitrobenzene (P99.0%) were obtained from Sigma Aldrich. The 2-propanol (P99.8%) was purchased from Merck Co. The prepared powders were analyzed by the ATR method with a VERTEX-70 infrared spectrometer to study their FTIR spectra. The XRD measurement was carried out using PANalytical Empyrean powder diffractometer by Cu Ka radiation. Also, the EDAX analysis was used for elemental analysis which model and detector type were S4800(I) and 7747/17-ME respectively. The TEM measurement was performed by a Hitachi HT7700 instrument. 2.2. TiO2 nanotubes preparation The hydrothermal method has been used for the preparation of TiO2 NTs [10]. For this aim, 4 g of commercial anatase TiO2 powder was dispersed in NaOH aqueous solution (10 M) and transferred into a Teflon-lined autoclave. The autoclave was heated at 393 K for 24 h and then cooled to room temperature. The obtained mixture was washed with deionized water and HCl solution (0.1 M) till the solution reached the pH = 7. The prepared TiO2 NTs were dried in a vacuum oven at 353 K, followed by calcination at 573 K for 4 h in the furnace. 2.3. Preparation of AuNPs/HPW/TiO2 NTs Firstly, HPW was deposited on the TiO2 NTs surface by dispersion of 100 mg TiO2 in 40 mL solution of HPW (10 mM), followed by stirring at room temperature for 24 h. It was separated by centrifugation (4000 rpm), washed with water three times and dried in an oven for 8 h at 60 °C. This composite is called HPW/TiO2-NTs. Then, for the decoration of obtained composite, 1 mL HAuCl4 solution (103 M) was added to 20 mL of dispersed obtained composite (20 mg/mL), and mixed with 1 mL 2-propanol in a quartz reactor cell. The suspension was purged with nitrogen gas for 15 min. Afterwards, the mixture was irradiated by a 15 W low pressure mercury vapor lamp, as UV light
2.4. Photocatalytic degradation of nitrobenzene The photocatalytic activity of AuNPs/HPW/TiO2-NTs nanocomposite film was evaluated by the degradation of nitrobenzene aqueous solution under visible light irradiation. For the photocatalytic reaction, 20 mg of nanocomposite was added to the nitrobenzene solution (40 ppm), purged with nitrogen for 15 min and magnetically stirred in the dark for 30 min to reach adsorption/desorption equilibrium. During the photoreaction, the samples were collected at given time intervals and centrifuged (4000 rpm) to separate the nanocomposite particles. Then, the solution was subsequently analyzed by UV–vis spectroscopy (A JASCO V-670 spectrophotometer (Japan)) at kmax = 268 nm, through Beer–Lambert law in which the absorbance value versus concentration obeys a linear relationship [29]. 3. Results and discussion
hm
½PW12 O40 3 =TiO2 -NTs þ ðCH3 Þ2 CHOH ! ½PW12 O40 4 =TiO2 -NTs þ ðCH3 Þ2 C@O þ 2Hþ
ð1Þ
[PW12O40]4 is highly active to transfer electrons efficiently to metal ions, and acts as reducing agent for the reduction of metal ions. So, in the contact with the gold ions, the [PW12O40]4 simultaneously oxidize to [PW12O40]3 and gold nanoparticles form according to Eq. (2):
½PW12 O40 4 =TiO2 -NTs þ Au3þ ! Au0 =½PW12 O40 3 =TiO2 -NTs
ð2Þ
Actually, the HPW plays the role of reducing agent and molecular bridge between AuNPs and TiO2 NTs. 3.2. Nanocomposite characterization The binding of HPW and AuNPs to TiO2 were followed by FTIR analysis, as shown in Fig. 1. As it can be seen, the TiO2 NTs does not have any significant band in the range of 600–2000 cm1. On the other hand, the HPW (H3PW12O40), as Keggin structure of POMs, consists of a cage of tungsten atoms which are linked by oxygen atoms and a phosphorus atom is at the center of the tetrahedral [33]. There are four main peaks inferred signatures due to the oxygen atoms which form four chemically distinct bonds. The 1080 cm1 is related to the PAO at the center of the HPW structure, 800 and 897 cm1 corresponding to the vibration modes of WAOaAW due to the oxygen atoms along the edges and WAObAW due to oxygen atoms at the corners of Keggin structure respectively. Also, the W@O corresponds to the asymmetric stretching of terminal oxygen atoms reflected at 971 cm1 [34]. By deposition of HPW to the surface of TiO2 NTs (HPW/TiO2-NTs), a red shift in the WAObAW bending vibrations appeared from
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Fig. 1. FTIR spectra of (a) prepared TiO2 NTs (b) pristine HPW, (c) HPW/TiO2-NTs, (d) Au/HPW/TiO2-NTs.
ca. 897 to ca. 891 cm1 and a blue shift can be observed in the vibration mode of W@O is observed from 971 to ca. 985, without any noticeable shift in other bending vibrational modes. These shifts confirmed the interaction of HPW to the TiO2 nanotubes surface through the oxygen atoms at the corners.
By decoration of HPW/TiO2-NTs surface by Au3+ ions, further significant red shifts appeared in the peaks of symmetric stretching vibrations of terminal oxygen atoms (W@O) from 985 to 952 cm1 and oxygen atom in PAO bond from 1080 to 1047 cm1. These suggest that the AuNPs bonded to HPW through terminal the oxygen atoms and oxygen atom which bind to the central phosphorus. The XRD (Fig. 2) was employed to analyze the crystalline phase of Au/HPW/TiO2-NTs nanocomposite. There are two distinct phases can be identified for TiO2 NTs, major component in anatase form, and smaller fraction in rutile form. Also, there is a minor fraction of gold phase by the peaks emerging at 2h = 38.4, 44.3, 64.7 and 77.88, which can be attributed to the diffraction peaks of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) faces of polycrystalline Au respectively [35]. These results confirmed the formation of metallic gold particles in the nanocomposite. The crystal size of Au particles is rather tricky to evaluate, as its peaks are severely overlapped with peaks originating from TiO2 NTs. On the other hand, the HPW did not change the intensity of diffraction peaks arising from TiO2 NTs which might be due to the formation of only a thin coating of HPW on the TiO2 surface [32]. The EDAX analysis was used to study the elemental composition of prepared nanocomposite as well as to determine the loading amount of AuNPs on the surface of nanocomposite. The spectrum and the elemental analysis results are shown in Fig. 3. As it can be seen, the presence of characteristic energy line at 1.7 keV corresponding to W MR (3.44 wt%) confirms the binding of HPW to the TiO2 surface. Moreover, it demonstrated the presence of gold metal in nanocomposites which is shown in characteristic energy line at 2.29 keV corresponding to Au Ma. Fig. 4 shows the TEM images of prepared TiO2 NTs and AuNPs/HPW/TiO2-NTs nanocomposite. Fig. 4a shows the nanotubes with the length of >200 nm and diameter of 20 nm. As it can be seen in Fig. 4b, the decorated quasi-spherical AuNPs (particles in dark contrasts); dispersed on the surface of TiO2 NTs without aggregation and their size are about 10–20 nm.
Fig. 2. XRD patterns of prepared Au/HPW/TiO2-NTs nanocomposite.
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Fig. 3. The EDAX spectra and elemental analysis of prepared nanocomposite.
Fig. 4. TEM image of prepared (a) TiO2 NTS and (b) AuNPs/HPW/TiO2-NTs nanocomposite.
3.3. Photocatalytic nitrobenzene removal by Au/HPW/TiO2-NTs The photocatalytic performance of Au/HPW/TiO2-NTs nanocomposite was investigated using the degradation of aqueous nitrobenzene solution under visible light irradiation. For comparison, the photocatalytic activity of TiO2 NTs, HPW, HPW/TiO2-NTs were also tested under the same conditions. The relative decrease of the nitrobenzene concentration, C/C0 against illumination time is shown in Fig. 5, in which C0 and C are the initial concentration of nitrobenzene and its concentration after t time respectively. As it can be seen, the photocatalytic performance changed in order of: HPW < TiO2 < HPW/TiO2 < Au/HPW/TiO 2 -NTs. Actually, the photocatalytic activity of HWP/TiO2 significantly increased by deposition of AuNPs. In this regard, the Langmuir–Hinshelwood model which is usually applied to describe the kinetics of photocatalytic reactions of aquatic organics has been used [36].
Also, the nitrobenzene degradation during photoexcitation could be easily followed by the kinetic rate of reaction in first 240 min, by the following expression:
r¼
dC kr K a C ¼ dt 1 þ K a C
ð3Þ
where r is the rate of photodegradation (mg min1), C is the organic concentration in solution (mg L1), t is the irradiation time (min), and kr and Ka are the reaction rate constant (mg L1 min1) and adsorption constant (L mg1) respectively. At low organic concentration (KaC 1), Eq. (3) can be simplified to Eq. (4) which resulted to a classical pseudo-first-order kinetic equation (Eq. (5)) [37]:
dC ¼ kr K a C dt C ln ¼ kr K a t ¼ K app t C0
ð4Þ ð5Þ
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Fig. 5. Photodegradation degradation of nitrobenzene in the presence of photocatalyst under visible light irradiation ([Nitrobenzene] = 40 ppm, photocatalyst dosage = 200 mg L1).
Table 1 The calculated kinetic rate of nitrobenzene (40 ppm) photodegradation in the presence of photocatalysts. Photocatalyst
Kapp
R2
HPW TiO2-NTs HPW/TiO2-NTs AuNPs/HPW/TiO2-NTs
0.0004 0.0019 0.0023 0.0078
0.9911 0.9882 0.9303 0.9961
where Kapp is the apparent first-order rate constant. The Kapp can be easily calculated in the plot of ln(C/C0) versus t. The calculated Kapp for the photocatalysis degradation of nitrobenzene are listed in Table 1. Obviously, the values of the rate constants are in good agreement with the degradation efficiencies of nitrobenzene and the Au/HPW/TiO2-NTs nanocomposite shows a highest rate constant. The binding of HPW to the TiO2-NTs surface resulted in small increase in the photodegradation nitrobenzene which might be
due to the cocatalytic activity of HPW and TiO2 which is agreement with the Sun et al. study [7]. By introducing of AuNPs, the photocatalytic performance significantly enhanced indicating that the photocatalytic rate using Au/HPW/TiO2-NTs nanocomposites was 4.1-fold increase in comparison to anatase TiO2.
3.4. Possible photocatalytic mechanism The possible photocatalytic mechanism of nitrobenzene degradation in the presence of AuNPs/HPW/TiO2-NTs under light irradiation is shown in Fig. 6. The weak photocatalytic activity of TiO2 NTs is due to the fast electron-hole recombination in the TiO2. The excitation of TiO2 semiconductor by incident photons with energy greater than or equal to TiO2 band gap energy leads to the promotion of electrons from the valence band (VB) of TiO2 into the conduction band (CB), across the band gap, where a large driving force exists to recombine the electron and newly generated hole [38]. The holes are
Fig. 6. Proposed scheme for possible photocatalytic mechanism of nitrobenzene degradation in the presence of AuNPs/HPW/TiO2-NTs under light irradiation.
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scavenged by water molecules or surface hydroxyl groups and then generate hydroxyl radicals (AOH). This highly reactive hydroxyl radicals could break the CAC bond in nitrobenzene and degrade it to some open-ring species and eventually CO2 and H2O [7]. By addition of HPW layer as a redox molecule to the surface of TiO2, HPW can act as the efficient electron trappers [39]. By the light irradiation, the electrons in CB of TiO2 can transfer to the empty d orbits of HPW and reduces the HPW to HPW⁄. Then, the HPW⁄ is reoxidized to HPW by transferring electrons to oxidants such as oxygen molecules. Also, the electron transfer can promote the charge separation which retard the fast charge-pair recombination on the TiO2 surface. It is demonstrated that the HPW acts as ‘‘electron scavenger” to transfer the photogenerated electrons and contributes to the suppression of charge recombination [28]. On the other hand, because of photoactivity of HPW molecules and their very strong electron transferability, the HPW bridging layer between TiO2 and AuNPs may provide an additional driving force to facilitate the charge transfer between them. The HPW acts as a redox molecule with high electron transfer ability between AuNPs and TiO2 surface and the charge recombination process could be blocked further [32]. Indeed, two phenomena can happen by the light excitation of nanocomposite: (i) the photon-mediated separation of holes and electrons within TiO2 and (ii) the UV-mediated reduction of HPW molecules between the TiO2 surface and AuNPs [32]. By migration of electron to from TiO2 to HPW (HPW⁄), the connected AuNPs to the HPW molecules will act as an electron sink. So, the transferred electrons from TiO2 to HPW⁄ will transfer from HPW⁄ to the AuNPs and the molecules is regenerated during the process, continuously. Also, the enhancement of photocatalytic activity of AuNPs/HPW/TiO2-NTs can be attributed to the different Fermi levels [40]. Usually, the energy of Fermi level of noble metals is lower than the CB edge of TiO2. So, the photo-promoted electrons can be captured by the noble metals, while photo produced holes remain in the semiconductor valence band [41]. The difference in the Fermi levels of gold and TiO2 make a continuous electron transfer from titaniato AuNPs to achievement of the same Fermi energy levels is achieved. Despite the effectiveness of this nanocomposite in photodegradation of nitrobenzene, it seems quite expensive for such a process. While nanotechnology techniques are truly emerging fields helping to achieve the next industrial revolution, it seems that the process must be studied more to make it feasible, economically [42]. 4. Conclusion In the present work, HPW was used highly localized UV-switchable reducing agent and multifunctional photocatalyst linker molecule to decorate the TiO2 nanotubes with gold nanoparticles. The characterization analysis confirmed the binding of HPW to the surface of TiO2 NTs and AuNPs. The photocatalytic performance of prepared Au/HPW/TiO2-NTs was measured in the degradation of nitrobenzene and its results confirmed the high activity of this triple photocatalysts system. Our results show that the photocatalytic performance changed in order of: HPW < TiO2 < HPW/TiO2 < Au/HPW/TiO2-NTs, whereas the kinetic rate of Au/HPW/TiO2-NT was 4.1 and 3.39 times more than those of TiO2 and HPW/TiO2 respectively. The possible mechanism involved in improving the photocatalytic performance of these materials has been proposed, in which the bridging layer of photoactive HPW, with strong electron transferability, between TiO2 NTs and AuNPs, may provide an additional driving force to facilitate the charge transfer between them.
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Photocatalytic degradation of nitrobenzene by gold nanoparticles decorated polyoxometalate immobilized TiO2 nanotubes Ali Ayati a,b,⇑, Bahareh Tanhaei b, Fatemeh F. Bamoharram c, Ali Ahmadpour d, Philipp Maydannik a, Mika Sillanpää a,e a
Laboratory of Green Chemistry, LUT School of Engineering Science, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland Department of Chemical Engineering, Quchan University of Advanced Technology, Quchan, Iran Department of Chemistry, Mashhad Branch-Islamic Azad University, Mashhad, Iran d Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran e Department of Civil and Environmental Engineering, Florida International University, Miami, FL 33174, USA b c
a r t i c l e
i n f o
Article history: Received 15 February 2016 Received in revised form 19 June 2016 Accepted 16 July 2016 Available online 18 July 2016 Keywords: TiO2 nanotube Polyoxometalate Gold nanoparticle Nitrobenzene Photodegradation
a b s t r a c t In this study, the gold nanoparticles were deposited on the surface of tungstophosphoric acid immobilized TiO2 nanotubes, in which tungstophosphoric acid plays the roles of highly localized UV-switchable reducing agent and multifunctional photocatalyst linker molecule. The prepared novel nanocomposite was characterized with FTIR, XRD, EDX and TEM and showed high photocatalytic efficiency in nitrobenzene removal. The results demonstrated that by introducing of AuNPs, the photocatalytic performance significantly enhanced, where the photocatalytic rate of using Au/HPW/TiO2-NTs nanocomposites was 4.1-fold increase compared to TiO2 nanotubes. The photocatalytic mechanism was proposed. Ó 2016 Published by Elsevier B.V.
1. Introduction Nitrobenzene (NB) as a highly toxic aromatic compound, is frequently discharged from the petroleum industry, dyes, herbicides, fungicides, wood and explosives manufacture effluents [1–3]. It is a skin and eyes irritant and affects the central nervous and cardiovascular systems and also, is suspected to be possible carcinogenic or mutagenic agent [3]. The maximum allowable concentration is reported 1 mg L1 in wastewaters [4]. In this regard, several treatment processes were reported for the nitrobenzene effective removal, such as ozonation [5], adsorption [6], catalytic [1] and photocatalytic degradation [7]. Among these methods, the latter process is largely studied due to the complete mineralization of organic pollutants to nontoxic or significantly less toxic inorganic compounds. Design of effective hybrid materials using the semiconductors has attracted growing interest for photocatalysis [8,9]. Titanium dioxide (TiO2) is one of the most widely studied semiconductor which has unique chemical and physical properties including its ⇑ Corresponding author at: Laboratory of Green Chemistry, LUT School of Engineering Science, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland. E-mail addresses: [email protected], [email protected] (A. Ayati). http://dx.doi.org/10.1016/j.seppur.2016.07.015 1383-5866/Ó 2016 Published by Elsevier B.V.
availability, low cost, nontoxicity, small crystal size, high specific surface area, highly porous structure [10–12]. TiO2 is well-known to exhibit photocatalytic activity due to the photo-generated charge carriers [10,13]. It is demonstrated that the TiO2 nanotubes as nanoparticulated forms of TiO2 possess higher photocatalytic activity in comparison to colloidal forms of TiO2, due to large specific surface area and pore volume [10,14] that can significantly increase the number of reaction sites. The most important drawbacks of bulk TiO2 and also, TiO2 nanotubes are their large band gap (Eg: 3.0–3.2 eV) which requires its exposure to ultraviolet irradiation for potential photocatalysis applications and its massive recombination rate of photogenerated electron(e)–hole (h+) pair which hinders its photocatalytic efficiency [15,16]. It is of great interest to develop TiO2 based photocatalyst with improved photocatalytic activity. Doping the TiO2 surface with noble metals, which act as electron acceptors, is one effective strategy to improve its photocatalytic activity. Among the noble metals, gold NPs deposition has attracted significant interest [17,18]. The charge separation between the excited electron (e) and hole (h+) resulting the strong interaction between gold and TiO2 [19] which decreases the band gap energy of TiO2 and enhances its photocatalytic activity [18,20]. On the other hand, it is demonstrated that the combination of the Polyoxometalate (POMs) and TiO2 can enhance the
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photocatalytic efficiency of TiO2-catalyzed reaction, due to the synergistic photocatalytic effect between these two components [21,22]. POMs as a molecularly defined inorganic metal–oxide clusters [23] can be easily reduced by photo/electro chemical procedures and their structures remain unchanged under stepwise and multielectron redox reactions. Because of these properties, they have been used as efficient photocatalysts, mild reductants and stabilizers in the green synthesis of metal NPs [24–27]. In our recent study [28], we applied Preyssler acid, as a POM structure, for the locally deposition of gold NPs on the TiO2 surface, wherein it played the role of interlink between the AuNPs and TiO2. The local decoration of AuNPs on the TiO2 surface prevents the formation of free non linked AuNPs in the solution and also, the formation of a novel nanocomposite composed of AuNPs, TiO2 and POM. In this study, we investigated the deposition of AuNPs on the tungstophosphoric acid (H3PW12O40, HPW) immobilized TiO2 nanotubes surface. The HPW (a Keggin type of POM) have attracted significant attention in catalysis and photocatalysis due to its low cost, low environmental impact, interesting electron and proton transfer, high thermal stability and storage characteristics [23]. We studied the photocatalytic activity of prepared nanocomposite for the degradation of nitrobenzene under simulated visible light irradiation.
source, under continuous stirring for an hour. The color of suspension was changed from milky to purple, indicating the formation of gold NPs. The obtained nanocomposite was separated using centrifugation (4000 rpm), washed with water and dried at 60 °C for 6 h.
2. Experimental
3.1. Mechanism of nanocomposite formation
2.1. Chemicals and instruments
The present HPW molecules bonded to TiO2 NTs in the HPW/TiO2-NTs composites were used for local reduction of gold ions on the surface of TiO2 NTs. By the UV irradiation of HPW/TiO2-NTs composites in the presence of 2-propanol, the bounded [PW12O40]3 can be easily reduced to [PW12O40]4 by the following equation [30–32]:
Titanium dioxide (TiO2, anatase, 99.8%), Tetrachloroauric acid (HAuCl4H2O, P99.9%), HPW and nitrobenzene (P99.0%) were obtained from Sigma Aldrich. The 2-propanol (P99.8%) was purchased from Merck Co. The prepared powders were analyzed by the ATR method with a VERTEX-70 infrared spectrometer to study their FTIR spectra. The XRD measurement was carried out using PANalytical Empyrean powder diffractometer by Cu Ka radiation. Also, the EDAX analysis was used for elemental analysis which model and detector type were S4800(I) and 7747/17-ME respectively. The TEM measurement was performed by a Hitachi HT7700 instrument. 2.2. TiO2 nanotubes preparation The hydrothermal method has been used for the preparation of TiO2 NTs [10]. For this aim, 4 g of commercial anatase TiO2 powder was dispersed in NaOH aqueous solution (10 M) and transferred into a Teflon-lined autoclave. The autoclave was heated at 393 K for 24 h and then cooled to room temperature. The obtained mixture was washed with deionized water and HCl solution (0.1 M) till the solution reached the pH = 7. The prepared TiO2 NTs were dried in a vacuum oven at 353 K, followed by calcination at 573 K for 4 h in the furnace. 2.3. Preparation of AuNPs/HPW/TiO2 NTs Firstly, HPW was deposited on the TiO2 NTs surface by dispersion of 100 mg TiO2 in 40 mL solution of HPW (10 mM), followed by stirring at room temperature for 24 h. It was separated by centrifugation (4000 rpm), washed with water three times and dried in an oven for 8 h at 60 °C. This composite is called HPW/TiO2-NTs. Then, for the decoration of obtained composite, 1 mL HAuCl4 solution (103 M) was added to 20 mL of dispersed obtained composite (20 mg/mL), and mixed with 1 mL 2-propanol in a quartz reactor cell. The suspension was purged with nitrogen gas for 15 min. Afterwards, the mixture was irradiated by a 15 W low pressure mercury vapor lamp, as UV light
2.4. Photocatalytic degradation of nitrobenzene The photocatalytic activity of AuNPs/HPW/TiO2-NTs nanocomposite film was evaluated by the degradation of nitrobenzene aqueous solution under visible light irradiation. For the photocatalytic reaction, 20 mg of nanocomposite was added to the nitrobenzene solution (40 ppm), purged with nitrogen for 15 min and magnetically stirred in the dark for 30 min to reach adsorption/desorption equilibrium. During the photoreaction, the samples were collected at given time intervals and centrifuged (4000 rpm) to separate the nanocomposite particles. Then, the solution was subsequently analyzed by UV–vis spectroscopy (A JASCO V-670 spectrophotometer (Japan)) at kmax = 268 nm, through Beer–Lambert law in which the absorbance value versus concentration obeys a linear relationship [29]. 3. Results and discussion
hm
½PW12 O40 3 =TiO2 -NTs þ ðCH3 Þ2 CHOH ! ½PW12 O40 4 =TiO2 -NTs þ ðCH3 Þ2 C@O þ 2Hþ
ð1Þ
[PW12O40]4 is highly active to transfer electrons efficiently to metal ions, and acts as reducing agent for the reduction of metal ions. So, in the contact with the gold ions, the [PW12O40]4 simultaneously oxidize to [PW12O40]3 and gold nanoparticles form according to Eq. (2):
½PW12 O40 4 =TiO2 -NTs þ Au3þ ! Au0 =½PW12 O40 3 =TiO2 -NTs
ð2Þ
Actually, the HPW plays the role of reducing agent and molecular bridge between AuNPs and TiO2 NTs. 3.2. Nanocomposite characterization The binding of HPW and AuNPs to TiO2 were followed by FTIR analysis, as shown in Fig. 1. As it can be seen, the TiO2 NTs does not have any significant band in the range of 600–2000 cm1. On the other hand, the HPW (H3PW12O40), as Keggin structure of POMs, consists of a cage of tungsten atoms which are linked by oxygen atoms and a phosphorus atom is at the center of the tetrahedral [33]. There are four main peaks inferred signatures due to the oxygen atoms which form four chemically distinct bonds. The 1080 cm1 is related to the PAO at the center of the HPW structure, 800 and 897 cm1 corresponding to the vibration modes of WAOaAW due to the oxygen atoms along the edges and WAObAW due to oxygen atoms at the corners of Keggin structure respectively. Also, the W@O corresponds to the asymmetric stretching of terminal oxygen atoms reflected at 971 cm1 [34]. By deposition of HPW to the surface of TiO2 NTs (HPW/TiO2-NTs), a red shift in the WAObAW bending vibrations appeared from
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Fig. 1. FTIR spectra of (a) prepared TiO2 NTs (b) pristine HPW, (c) HPW/TiO2-NTs, (d) Au/HPW/TiO2-NTs.
ca. 897 to ca. 891 cm1 and a blue shift can be observed in the vibration mode of W@O is observed from 971 to ca. 985, without any noticeable shift in other bending vibrational modes. These shifts confirmed the interaction of HPW to the TiO2 nanotubes surface through the oxygen atoms at the corners.
By decoration of HPW/TiO2-NTs surface by Au3+ ions, further significant red shifts appeared in the peaks of symmetric stretching vibrations of terminal oxygen atoms (W@O) from 985 to 952 cm1 and oxygen atom in PAO bond from 1080 to 1047 cm1. These suggest that the AuNPs bonded to HPW through terminal the oxygen atoms and oxygen atom which bind to the central phosphorus. The XRD (Fig. 2) was employed to analyze the crystalline phase of Au/HPW/TiO2-NTs nanocomposite. There are two distinct phases can be identified for TiO2 NTs, major component in anatase form, and smaller fraction in rutile form. Also, there is a minor fraction of gold phase by the peaks emerging at 2h = 38.4, 44.3, 64.7 and 77.88, which can be attributed to the diffraction peaks of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) faces of polycrystalline Au respectively [35]. These results confirmed the formation of metallic gold particles in the nanocomposite. The crystal size of Au particles is rather tricky to evaluate, as its peaks are severely overlapped with peaks originating from TiO2 NTs. On the other hand, the HPW did not change the intensity of diffraction peaks arising from TiO2 NTs which might be due to the formation of only a thin coating of HPW on the TiO2 surface [32]. The EDAX analysis was used to study the elemental composition of prepared nanocomposite as well as to determine the loading amount of AuNPs on the surface of nanocomposite. The spectrum and the elemental analysis results are shown in Fig. 3. As it can be seen, the presence of characteristic energy line at 1.7 keV corresponding to W MR (3.44 wt%) confirms the binding of HPW to the TiO2 surface. Moreover, it demonstrated the presence of gold metal in nanocomposites which is shown in characteristic energy line at 2.29 keV corresponding to Au Ma. Fig. 4 shows the TEM images of prepared TiO2 NTs and AuNPs/HPW/TiO2-NTs nanocomposite. Fig. 4a shows the nanotubes with the length of >200 nm and diameter of 20 nm. As it can be seen in Fig. 4b, the decorated quasi-spherical AuNPs (particles in dark contrasts); dispersed on the surface of TiO2 NTs without aggregation and their size are about 10–20 nm.
Fig. 2. XRD patterns of prepared Au/HPW/TiO2-NTs nanocomposite.
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Fig. 3. The EDAX spectra and elemental analysis of prepared nanocomposite.
Fig. 4. TEM image of prepared (a) TiO2 NTS and (b) AuNPs/HPW/TiO2-NTs nanocomposite.
3.3. Photocatalytic nitrobenzene removal by Au/HPW/TiO2-NTs The photocatalytic performance of Au/HPW/TiO2-NTs nanocomposite was investigated using the degradation of aqueous nitrobenzene solution under visible light irradiation. For comparison, the photocatalytic activity of TiO2 NTs, HPW, HPW/TiO2-NTs were also tested under the same conditions. The relative decrease of the nitrobenzene concentration, C/C0 against illumination time is shown in Fig. 5, in which C0 and C are the initial concentration of nitrobenzene and its concentration after t time respectively. As it can be seen, the photocatalytic performance changed in order of: HPW < TiO2 < HPW/TiO2 < Au/HPW/TiO 2 -NTs. Actually, the photocatalytic activity of HWP/TiO2 significantly increased by deposition of AuNPs. In this regard, the Langmuir–Hinshelwood model which is usually applied to describe the kinetics of photocatalytic reactions of aquatic organics has been used [36].
Also, the nitrobenzene degradation during photoexcitation could be easily followed by the kinetic rate of reaction in first 240 min, by the following expression:
r¼
dC kr K a C ¼ dt 1 þ K a C
ð3Þ
where r is the rate of photodegradation (mg min1), C is the organic concentration in solution (mg L1), t is the irradiation time (min), and kr and Ka are the reaction rate constant (mg L1 min1) and adsorption constant (L mg1) respectively. At low organic concentration (KaC 1), Eq. (3) can be simplified to Eq. (4) which resulted to a classical pseudo-first-order kinetic equation (Eq. (5)) [37]:
dC ¼ kr K a C dt C ln ¼ kr K a t ¼ K app t C0
ð4Þ ð5Þ
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Fig. 5. Photodegradation degradation of nitrobenzene in the presence of photocatalyst under visible light irradiation ([Nitrobenzene] = 40 ppm, photocatalyst dosage = 200 mg L1).
Table 1 The calculated kinetic rate of nitrobenzene (40 ppm) photodegradation in the presence of photocatalysts. Photocatalyst
Kapp
R2
HPW TiO2-NTs HPW/TiO2-NTs AuNPs/HPW/TiO2-NTs
0.0004 0.0019 0.0023 0.0078
0.9911 0.9882 0.9303 0.9961
where Kapp is the apparent first-order rate constant. The Kapp can be easily calculated in the plot of ln(C/C0) versus t. The calculated Kapp for the photocatalysis degradation of nitrobenzene are listed in Table 1. Obviously, the values of the rate constants are in good agreement with the degradation efficiencies of nitrobenzene and the Au/HPW/TiO2-NTs nanocomposite shows a highest rate constant. The binding of HPW to the TiO2-NTs surface resulted in small increase in the photodegradation nitrobenzene which might be
due to the cocatalytic activity of HPW and TiO2 which is agreement with the Sun et al. study [7]. By introducing of AuNPs, the photocatalytic performance significantly enhanced indicating that the photocatalytic rate using Au/HPW/TiO2-NTs nanocomposites was 4.1-fold increase in comparison to anatase TiO2.
3.4. Possible photocatalytic mechanism The possible photocatalytic mechanism of nitrobenzene degradation in the presence of AuNPs/HPW/TiO2-NTs under light irradiation is shown in Fig. 6. The weak photocatalytic activity of TiO2 NTs is due to the fast electron-hole recombination in the TiO2. The excitation of TiO2 semiconductor by incident photons with energy greater than or equal to TiO2 band gap energy leads to the promotion of electrons from the valence band (VB) of TiO2 into the conduction band (CB), across the band gap, where a large driving force exists to recombine the electron and newly generated hole [38]. The holes are
Fig. 6. Proposed scheme for possible photocatalytic mechanism of nitrobenzene degradation in the presence of AuNPs/HPW/TiO2-NTs under light irradiation.
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scavenged by water molecules or surface hydroxyl groups and then generate hydroxyl radicals (AOH). This highly reactive hydroxyl radicals could break the CAC bond in nitrobenzene and degrade it to some open-ring species and eventually CO2 and H2O [7]. By addition of HPW layer as a redox molecule to the surface of TiO2, HPW can act as the efficient electron trappers [39]. By the light irradiation, the electrons in CB of TiO2 can transfer to the empty d orbits of HPW and reduces the HPW to HPW⁄. Then, the HPW⁄ is reoxidized to HPW by transferring electrons to oxidants such as oxygen molecules. Also, the electron transfer can promote the charge separation which retard the fast charge-pair recombination on the TiO2 surface. It is demonstrated that the HPW acts as ‘‘electron scavenger” to transfer the photogenerated electrons and contributes to the suppression of charge recombination [28]. On the other hand, because of photoactivity of HPW molecules and their very strong electron transferability, the HPW bridging layer between TiO2 and AuNPs may provide an additional driving force to facilitate the charge transfer between them. The HPW acts as a redox molecule with high electron transfer ability between AuNPs and TiO2 surface and the charge recombination process could be blocked further [32]. Indeed, two phenomena can happen by the light excitation of nanocomposite: (i) the photon-mediated separation of holes and electrons within TiO2 and (ii) the UV-mediated reduction of HPW molecules between the TiO2 surface and AuNPs [32]. By migration of electron to from TiO2 to HPW (HPW⁄), the connected AuNPs to the HPW molecules will act as an electron sink. So, the transferred electrons from TiO2 to HPW⁄ will transfer from HPW⁄ to the AuNPs and the molecules is regenerated during the process, continuously. Also, the enhancement of photocatalytic activity of AuNPs/HPW/TiO2-NTs can be attributed to the different Fermi levels [40]. Usually, the energy of Fermi level of noble metals is lower than the CB edge of TiO2. So, the photo-promoted electrons can be captured by the noble metals, while photo produced holes remain in the semiconductor valence band [41]. The difference in the Fermi levels of gold and TiO2 make a continuous electron transfer from titaniato AuNPs to achievement of the same Fermi energy levels is achieved. Despite the effectiveness of this nanocomposite in photodegradation of nitrobenzene, it seems quite expensive for such a process. While nanotechnology techniques are truly emerging fields helping to achieve the next industrial revolution, it seems that the process must be studied more to make it feasible, economically [42]. 4. Conclusion In the present work, HPW was used highly localized UV-switchable reducing agent and multifunctional photocatalyst linker molecule to decorate the TiO2 nanotubes with gold nanoparticles. The characterization analysis confirmed the binding of HPW to the surface of TiO2 NTs and AuNPs. The photocatalytic performance of prepared Au/HPW/TiO2-NTs was measured in the degradation of nitrobenzene and its results confirmed the high activity of this triple photocatalysts system. Our results show that the photocatalytic performance changed in order of: HPW < TiO2 < HPW/TiO2 < Au/HPW/TiO2-NTs, whereas the kinetic rate of Au/HPW/TiO2-NT was 4.1 and 3.39 times more than those of TiO2 and HPW/TiO2 respectively. The possible mechanism involved in improving the photocatalytic performance of these materials has been proposed, in which the bridging layer of photoactive HPW, with strong electron transferability, between TiO2 NTs and AuNPs, may provide an additional driving force to facilitate the charge transfer between them.
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